Skittery as mercury on a silver tray, the political mood is pouring this way and that with prodigious speed. Elation tumbles into despair and triumph into calamity. Nothing is fixed and no prediction can be trusted; only that the expected has fallen apart and plans will have to be changed. The politician and the party that understands this best will emerge strengthened, when the hysteria falls away and the long march towards a general election in 2009 or perhaps even 2010 begins.

This march will require open minds and nimble feet, as well as great confidence in the destination. None of these were on show yesterday from Gordon Brown, defeated by David Cameron at a brutal prime minister's question time that seemed to catch the present political intensity. The queasy faces of cabinet ministers around Mr Brown, and the notable silence of Labour MPs behind him, reflected the scale of the misfortune which his equivocation over an early election has inflicted on his party. All the great confidence of the summer and the Bournemouth party conference has gone. It risks being replaced not with an energetic display of Labour's capabilities and ambitions, but an absence of clarity that would only deepen doubt. Mr Cameron was quick and shrill yesterday, which can be effective in the Commons as it is not in the country, but the prime minister had no answer to his taunts. His party conference speech, his statement on Iraq, and the brief and unconfident pre-budget report: none of these aid an understanding of the vision Mr Brown says he wants to set out. Yesterday in the Commons he had to fall back on old achievements of the Blair era to protect himself from Mr Cameron - the minimum wage, the independent Bank of England. "Who has been leading the debate in this country?" asked the prime minister, and the answer has been Labour, but his use of the past tense was telling. It was a retrospective answer from a party leader who must now look forward to win.

Win he can of course, and not only because the base of support for Labour remains strong. The party was ahead of the Conservatives in the last poll, largely carried out before news of the cancelled election broke. Meanwhile Liberal Democrat support, which has plunged disturbingly, might recover, more to Mr Cameron's disadvantage than Mr Brown's. Sir Menzies Campbell ended his party conference with a capable speech that should have strengthened his leadership, but unfair though it is, he has not caught the public mood and probably never will. How will his party respond? And how will Conservatives respond to their current elation, which could prove dangerous if Tory MPs mistake it for public support for tax cuts before better services, which the party has no clear idea of how to deliver? Mr Cameron's swag-full of policy reviews does not yet add up to a coherent idea for government and his party must use the time it now has to decide what its fluid talk of social responsibility really means.

Labour's opportunity is that it can act now, in power, not just to promise change but to demonstrate it. The waste of opportunity in the pre-budget report is what made it so disappointing. By borrowing ideas, rather than revealing new and better ones, it played an instant political game; and it created confusion too. Yesterday Mr Darling did not seem to know what parts of it meant. Does Labour plan a review of the way charging in social care works? Did it mean to exclude the children of divorced couples from the inheritance tax allowance given to married ones? This was the carelessness of a pressed government not the vigour of a focused one.

Soon, politics will grow calm again. Only then will it be possible to judge what impact the past few weeks have had on the public. Perhaps this has all been an adolescent drama, contained in Westminster, and Mr Brown's opening strength will return. After this week, however, he cannot be certain of such comfort.